This invention relates to thermoplastic elastomers having improved adhesive properties and processes for making the same.
Thermoplastic elastomers are known. They have many of the properties of thermoset elastomers, yet they are processable as thermoplastics. One type of thermoplastic elastomer is a thermoplastic vulcanizate, which may be characterized by finely-divided rubber particles dispersed within a plastic. These rubber particles are crosslinked to promote elasticity. Thermoplastic vulcanizates are conventionally produced by dynamic vulcanization, which is a process whereby a rubber is cured or vulcanized within a blend with at least one non-vulcanizing polymer while the polymers are undergoing mixing or masticating at some elevated temperature, preferably above the melt temperature of the non-vulcanizing polymer.
Conventional thermoplastic vulcanizations, however, are typically not useful as adhesives. The plastic phase, which is typically non-polar, generally serves as the matrix of the blend. As a result, the rich non-polar surface of the blend reduces its ability to adhere to metal or other polar substrates, including many engineering resins. The adhesion problem is further exacerbated as attempts are made to harden the thermoplastic vulcanizate. As those skilled in the art appreciate, thermoplastic vulcanizates can be hardened by increasing the plastic content. As a result, however, the amount of plastic (i.e., non-polar material) at the surface is increased, which thereby further reduces the ability of thermoplastic vulcanizate to adhere to polar substrates.
Several attempts to improve the adhesive properties of thermoplastic vulcanizates have been made. In one instance, functionalized polyolefins have been added. In another, copolymers of polyamides and functionalized polyolefins have been added to improve adhesion to polyamides. In another instance, block copolymers of functionalized olefins, thermoplastic polyurethane, copolyester or copolyamide, and isocyanates have been added.
While these attempts have made some advancements in the art, there is a continued need to find other solutions to the adhesion problem.
In general the present invention provides a thermoplastic vulcanizate prepared by dynamically vulcanizing a halogenated butyl rubber within a mixture that includes the halogenated butyl rubber and from about 10 to about 100 parts by weight of a crystalline thermoplastic resin per 100 parts by weight rubber, to thereby form a thermoplastic vulcanizate, where the thermoplastic resin includes a resin other than polyethylene, melt mixing the thermoplastic vulcanizate with about 5 to about 75 parts by weight of polyethylene resin per 100 parts by weight rubber, from about 3 to about 60 parts by weight of a tackifier per 100 parts by weight rubber, and from about 5 to about 250 parts by weight of a block copolymer containing at least the structure S-B/S-S per 100 parts by weight rubber, where the S block of the block copolymer has a glass transition temperature above about 25xc2x0 C., the B/S block has a glass transition temperature below about 25xc2x0 C., and the S content of the B/S block is from about 40 to about 60 percent by weight of said S/B block, the diene content of the total block copolymer is less than 40 weight percent, and the B/S block amount to at least 50 weight percent of the total block copolymer.
The present invention also includes a thermoplastic vulcanizate comprising a vulcanized rubber, a thermoplastic resin other than polyethylene, a polyethylene resin, a flexible block copolymer, and a tackifier.
The present invention further includes a process for forming a thermoplastic elastomer mixture comprising dynamically vulcanizing a rubber within a mixture that includes the rubber and a crystalline thermoplastic resin, to thereby form a thermoplastic vulcanizate, and mixing the thermoplastic vulcanizate with a block copolymer containing at least the structure S-S/B-S.
The addition of a flexible block copolymer together with a high density polyethylene and a tackifier to thermoplastic vulcanizates has advantageously led to the discovery that the adhesion of thermoplastic vulcanizates can be improved. Significantly, this improvement has led to technologically useful thermoplastic vulcanizates even at higher hardness levels. Furthermore, where butyl rubber is employed as the rubber component, and even more advantageously halogenated butyl rubber, the adhesive properties of the thermoplastic vulcanizate is greatly improved.